In typical wireless networks, such as in long-term evolution (LTE), the selection of shared physical channels for uplink (UL) data transmission is based on a scheduling (or grant) mechanism. Such a mechanism takes place at a base station (BS). In particular, a user equipment (UE) sends scheduling requests or grants to the BS on an UL control channel. Upon receiving the scheduling requests or grants, the BS transmits UL grants to the UE on a downlink (DL) control channel indicating the resource allocation for UL data transmission. Such resources may e.g. be time, frequency, code channels, etc. Thereafter, the UE transmit UL data on such granted resources.
In traditional contention based approaches transmission medium access is not controlled and thus transmissions using same physical resources can collide in a way that transmissions cannot be received successfully.
A conventional solution has been proposed which describes a scheme for grant-free UL data transmission. However, this conventional solution is a contention based scheme, similar to that used in IEEE 802.11 standard. In particular, in the conventional solution the network defines the physical resources for contention based UL data transmission in terms of time, frequency and/or code domain. This allows for sending data packets without scheduling and associated allocation signalling latency. However, contention based access schemes are inefficient in terms of usage of physical resources due to the high risk of colliding UL data transmissions. Hence, the conventional solution is not appropriate for users generating high traffic loads.
The limitation of schemes that are based on scheduling or grant request is the large signalling overhead, especially in cases where the UL data transmitted is small. As an example, for data packets as small as 20 bytes, the resources used by typical mechanisms based on scheduling or grant requests is around 30% or even 50%. Another limitation of such schemes is the high latency between the scheduling request and the acknowledged transmissions. For example, in LTE the UE first has to inform network about need for uplink resources by scheduling request in next possible occasion on uplink control channel. Then the UE has to wait for possible uplink allocation. Once the uplink allocation is received, the UE has to still wait for allocated subframe and acknowledgement after the transmission. Reduction of such latencies caused by such scheduling related signalling is critical for delay sensitive communications.
The limitations of contention based solutions are always associated with the high risk of UL data collisions. In case of UL data collision, data cannot be decoded correctly and retransmissions are needed. This increases the latency and makes contention based schemes unattractive for delay sensitive communications. Moreover, contention based mechanisms are inefficient in terms of usage of radio resources since the capacity for UL data transmission needs to be larger than that actually used in order to have a low collision probability. Due to this, contention based mechanisms are best suited for small occasional transmissions.